Conventionally, postage meters essentially comprise a rotating drum for printing fixed and variable franking data, and, where applicable, for printing advertising data; accounting means essentially provided for keeping the financial accounts of the meter as and when mail articles are franked; a mechanism for advancing articles to be franked and including at least one platen roller facing the drum; and control circuits causing the drum to rotate through one turn each time successive articles are to be franked, and controlling the accounting means accordingly.
The rotating print drum has both peripheral printing plates for printing fixed franking data and advertising data, and also print means for printing variable data, in particular the postage value and date. These print means are usually sets of wheels which are mounted to project slightly from the periphery of the drum, through suitable openings in the postal image printing plate, and which rotate with the drum. In a variant, the print means for printing the variable data may be constituted by a row of ink jet nozzles, mounted inside the drum and with the nozzles facing the platen being triggered when each opening in the postal image printing plate moves round to coincide with the nozzles.
Existing postage meters are either one-piece units, e.g. small meters referred to as "office portable" meters, or else they have a removable print head installed on a base, e.g. large meters for processing greater volumes of mail. For reasons of security against possible fraud, users are not allowed access to the inside of one-piece meters. For the same reasons, in a meter having a removable head, the head constitutes a secure portable assembly which contains, in particular, the drum and the accounting means together with the control circuits for controlling the accounting means. The base essentially includes a conveyor mechanism for advancing articles, a drive motor for this mechanism and the control means for controlling the drum, this control generally being taken from the conveyor mechanism motor via a controlled clutch.
A meter having a removable print head requires mechanical and electrical coupling means to be provided between the head and the base to drive the drum and to synchronize the commands of the circuits in the head and in the base. Such removable installation of the head also requires the head and base to be equipped with positioning means, so that the head can be installed accurately in its position on the base and so that the head and the base can be coupled together both mechanically and electrically.
Mechanical coupling is conventionally provided by means of a driving gear wheel in the base, which gear wheel projects a little into the location provided on the base for receiving the head, and by means of a gear wheel in the head, which gear wheel is carried by the drum drive shaft and also projects a little from beneath the head. Electrical coupling is provided by two complementary connectors, one in the base and one in the head.
Both mechanical and electrical coupling between the base and the head pose numerous practical problems. They make mechanical and electrical interface circuits complex. In particular, to achieve satisfactory mechanical coupling of the head on the base, it is necessary to achieve almost perfect centering of the head drum drive shaft relative to the base drive shaft and relative to the transmission mechanism in the base between this drive shaft and the driving gear wheel. This mechanical coupling requires a large number of parts in the base, and installation and positioning tolerances that are difficult to comply with.
An object of the present invention is to define a new design of removable print head on a corresponding base, to avoid the above-mentioned drawbacks and, furthermore, to avoid the risks of damaging the couplings between the base and the head, in particular during installation of the head. The invention thus makes it possible to standardize all ranges of postage meters, which can advantageously all be made with removable heads (including the meters in the small range, in which access to their circuits and mechanisms need only be prevented in their heads).